@article{bradshaw_samberg_carlin_colter_edmondson_hong_fetzer_karam_bedair_2014, title={GaInP/GaAs Tandem Solar Cells With InGaAs/GaAsP Multiple Quantum Wells}, volume={4}, ISSN={["2156-3403"]}, DOI={10.1109/jphotov.2013.2294750}, abstractNote={Lattice-matched multiple quantum wells (MQWs) consisting of InxGa1-xAs wells with very thin GaAs0.2P0.8 barriers have been incorporated into a GaInP/GaAs tandem solar cell. InGaAs/GaAsP MQWs increase the short-circuit current of the GaAs cell by extending the absorption range, with minimal impact on an open-circuit voltage, thus alleviating current matching restrictions placed by the GaAs cell on multijunction solar cells. MQWs with very thin, tensile strained, high phosphorus content GaAsP barriers allow tunneling to dominate carrier transport across the MQWs and balance the compressive strain of the InGaAs wells such that material quality remains high for subsequent top cell growth. We show that the addition of the QW layers enhances the GaAs cell, does not degrade the performance of the GaInP top cell, and leads to potential efficiency enhancements.}, number={2}, journal={IEEE JOURNAL OF PHOTOVOLTAICS}, author={Bradshaw, Geoffrey K. and Samberg, Joshua P. and Carlin, C. Zachary and Colter, Peter C. and Edmondson, Kenneth M. and Hong, William and Fetzer, Chris and Karam, Nasser and Bedair, Salah M.}, year={2014}, month={Mar}, pages={614–619} }
 @article{bradshaw_carlin_samberg_el-masry_colter_bedair_2013, title={Carrier Transport and Improved Collection in Thin-Barrier InGaAs/GaAsP Strained Quantum Well Solar Cells}, volume={3}, ISSN={["2156-3381"]}, DOI={10.1109/jphotov.2012.2216858}, abstractNote={Multiple quantum wells (MQW) lattice matched to GaAs consisting of In0.14Ga0.76As wells balanced with GaAs0.24P0.76 barriers have been used to extend the absorption of GaAs subcells to longer wavelengths for use in an InGaP/GaAs/Ge triple-junction photovoltaic cell. Thin barriers with high-phosphorus composition are capable of balancing the strain from the InGaAs wells; thus, creating conditions to allow for thicker wells and for carrier tunneling to dominate transport across the structure. As a result, a larger percentage of the depletion region is occupied by InGaAs quantum wells that absorb wavelengths beyond 875 nm and the indium composition is not limited by thermionic emission requirements. Measurements at elevated temperatures and reverse bias suggest that a thermally assisted tunneling mechanism is responsible for transport through the barriers.}, number={1}, journal={IEEE JOURNAL OF PHOTOVOLTAICS}, author={Bradshaw, Geoffrey K. and Carlin, C. Zachary and Samberg, Joshua P. and El-Masry, Nadia A. and Colter, Peter C. and Bedair, Salah M.}, year={2013}, month={Jan}, pages={278–283} }
 @inproceedings{bradshaw_carlin_samberg_colter_bedair_2013, title={Determination of carrier recombination lifetime in InGaAs quantum wells from external quantum efficiency measurements}, DOI={10.1109/pvsc.2013.6744143}, abstractNote={GaAs cells containing multiple quantum wells (MQW) of strained InGaAs/GaAsP can enhance efficiency in multijunction solar cells. Determination of carrier recombination lifetime in the InGaAs well is useful to understand material quality and carrier transport across the structure. GaAs p-i-n structures with and without strain balanced In0.17Ga0.83As wells and GaAs0.25P0.75 barriers were grown by MOCVD on p-type GaAs substrates. The GaAsP barrier thickness was varied between devices to intentionally influence carrier transport. A decrease in EQE was observed as barrier width was increased, which was attributed to an increase in tunneling lifetime, τtn. While this EQE decrease is undesirable in practical devices, it is useful for determining the recombination lifetime, τr, of the InGaAs wells. The decrease in EQE was observed only at wavelengths of light greater than 600 nm, indicating that minority carrier electrons generated in the base are responsible for the reduction in EQE. Shorter wavelengths (<;600 nm) of light are almost completely absorbed before reaching the base and primarily generate holes in the emitter. The tunneling lifetime and the currents generated in the p-i-n structures were modeled to calculate the EQE of a GaAs control and both thick and thin barrier MQW devices. The probability of transport through the entire MQW structure, Ptot, was varied until the calculated EQE fit the experimental data. The value of Ptot was then correlated to the only unknown parameter, the recombination lifetime. Using this method the recombination lifetime in In0.17Ga0.83As in the QW was determined to be 110 ns, which agrees with values found in previous time resolved photoluminescence measurements of metamorphic InGaAs films.}, booktitle={2013 ieee 39th photovoltaic specialists conference (pvsc)}, author={Bradshaw, G. K. and Carlin, C. Z. and Samberg, J. P. and Colter, P. C. and Bedair, S. M.}, year={2013}, pages={264–267} }
 @article{samberg_carlin_bradshaw_colter_harmon_allen_hauser_bedair_2013, title={Effect of GaAs interfacial layer on the performance of high bandgap tunnel junctions for multijunction solar cells}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4819917}, abstractNote={The effect of the heterojunction interface on the performance of high bandgap InxGa1−xP:Te/Al0.6Ga0.4As:C tunnel junctions (TJs) was investigated. The insertion of 30 A of GaAs:Te at the junction interface resulted in a peak current of 1000 A/cm2 and a voltage drop of ∼3 mV for 30 A/cm2 (2000× concentration). The presence of this GaAs interfacial layer also improved the uniformity across the wafer. Modeling results are consistent with experimental data and were used to explain the observed enhancement in TJ performance. This architecture could be used within multijunction solar cells to extend the range of usable solar concentration with minimal voltage drop.}, number={10}, journal={APPLIED PHYSICS LETTERS}, author={Samberg, Joshua P. and Carlin, C. Zachary and Bradshaw, Geoff K. and Colter, Peter C. and Harmon, Jeffrey L. and Allen, J. B. and Hauser, John R. and Bedair, S. M.}, year={2013}, month={Sep} }
 @article{samberg_carlin_bradshaw_colter_bedair_2013, title={Growth and Characterization of InxGa1-xAs/GaAs1-yPy Strained-Layer Superlattices with High Values of y (similar to 80%)}, volume={42}, ISSN={["0361-5235"]}, DOI={10.1007/s11664-012-2375-0}, abstractNote={Strained-layer superlattice (SLS) structures, such as InGaAs/GaAsP lattice matched to GaAs, have shown great potential in absorption devices such as photodetectors and triple-junction photovoltaic cells. However, until recently they have been somewhat hindered by their usage of low-phosphorus GaAsP barriers. High-P-composition GaAsP was developed as the barrier for InGaAs/GaAsP strained-layer superlattice (SLS) structures, and the merits of using such a high composition of phosphorus are discussed. It is believed that these barriers represent the highest phosphorus content to date in such a structure. By using high-composition GaAsP the carriers are collected via tunneling (for barriers ≤30 Å) as opposed to thermionic emission. Thus, by utilizing thin, high-content GaAsP barriers one can increase the percentage of the intrinsic in a p-i-n structure that is composed of InGaAs wells in addition to increasing the number of periods that can be grown for given depletion width. However, standard SLSs of this type inherently possess undesirable compressive strain and quantum size effects (QSEs) that cause the optical absorption of the thin InGaAs SLS wells to shift to higher energies relative to that of bulk InGaAs of the same composition. To circumvent these deleterious QSEs, stress-balanced, pseudomorphic InGaAs/GaAsP staggered SLSs were grown. Staggering was achieved by removing a portion of one well and adding it to an adjacent well. The spectral response obtained from device characterization indicated that staggering resulted in thicker InGaAs films with reduced cutoff energy. Additionally, these data confirm that tunneling is a very effective means for carrier transport in the SLS.}, number={5}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Samberg, J. P. and Carlin, C. Z. and Bradshaw, G. K. and Colter, P. C. and Bedair, S. M.}, year={2013}, month={May}, pages={912–917} }
 @article{samberg_alipour_bradshaw_carlin_colter_lebeau_el-masry_bedair_2013, title={Interface properties of Ga(As,P)/(In,Ga)As strained multiple quantum well structures}, volume={103}, ISSN={["0003-6951"]}, DOI={10.1063/1.4818548}, abstractNote={(In,Ga)As/Ga(As,P) multiple quantum wells (MQWs) with GaAs interface layers have been characterized with photoluminescence (PL) and high resolution scanning transmission electron microscopy (STEM). By growing (In,Ga)As/Ga(As,P) MQWs with asymmetric GaAs interfacial layers, we found that phosphorus carry-over had a profound effect on the absorption edge of the (In,Ga)As wells. Evidence for this phosphorus was initially determined via PL and then definitively proven through STEM and energy dispersive x-ray spectroscopy. We show that the phosphorus carry-over can be prevented with sufficiently thick GaAs transition layers. Preliminary results for GaAs p-i-n solar cells utilizing the improved MQWs are presented.}, number={7}, journal={APPLIED PHYSICS LETTERS}, author={Samberg, Joshua P. and Alipour, Hamideh M. and Bradshaw, Geoffrey K. and Carlin, C. Zachary and Colter, Peter C. and LeBeau, James M. and El-Masry, N. A. and Bedair, Salah M.}, year={2013}, month={Aug} }
 @article{carlin_bradshaw_samberg_colter_bedair_2013, title={Minority Carrier Transport and Their Lifetime in InGaAs/GaAsP Multiple Quantum Well Structures}, volume={60}, ISSN={["1557-9646"]}, DOI={10.1109/ted.2013.2268421}, abstractNote={Minority carrier transport across InGaAs/GaAsP multiple quantum wells is studied by measuring the response of p-i-n and n-i-p GaAs solar cell structures. It is observed that the spectral response depends critically upon the width of the GaAsP barriers and the device polarity. Electron tunneling is not as efficient as hole tunneling due to a higher conduction band barrier. The spectral response depends on the relative magnitude of the carrier lifetime as compared with the tunneling lifetime. This paper deduces an estimated electron lifetime of 110 ns in In0.14Ga0.86As wells and 25 ns in In0.17Ga0.83As wells, which agree with published results.}, number={8}, journal={IEEE TRANSACTIONS ON ELECTRON DEVICES}, author={Carlin, Conrad Zachary and Bradshaw, Geoffrey Keith and Samberg, Joshua Paul and Colter, Peter C. and Bedair, Salah M.}, year={2013}, month={Aug}, pages={2532–2536} }
 @inproceedings{hauser_carlin_harmon_bradshaw_samberg_colter_bedair_2013, title={Modeling an InGaP/AlGaAs tunnel junction containing an AlAs diffusion barrier}, DOI={10.1109/pvsc.2013.6744883}, abstractNote={Cost improvements in concentrated photovoltaic (CPV) systems can be achieved by operating at increased solar concentration. Current multijunction CPV systems are limited to about 1000× concentration by the performance of the tunnel junctions (TJ) which connect the subcells. The TJ requires materials which are doped in excess of 1019 cm-3 in order to operate effectively, and so are susceptible to diffusion during the growth of subsequent layers. This paper considers a tunnel junction comprised of tellurium doped n+-InGaP and carbon doped p+-AlGaAs with a several monolayers of AlAs at the interface. The diffusion profile of the dopants was found and used to calculate the tunneling current through a junction. Due to uncertainty in the diffusion constants of C and Te in the three layers, the tunneling current was calculated for several values of Dt. The diffusion constant ratio in the AlAs was taken as a fraction of the diffusion constant in the other two layers. A significant increase in peak tunneling current was seen for Dt>1×10-14 cm2 when a three monolayer thick AlAs barrier was present.}, booktitle={2013 ieee 39th photovoltaic specialists conference (pvsc)}, author={Hauser, J. and Carlin, Z. and Harmon, J. and Bradshaw, G. and Samberg, J. and Colter, P. and Bedair, S.}, year={2013}, pages={2082–2085} }
 @inproceedings{samberg_bradshaw_carlin_colter_edmondson_hong_fetzer_karam_el-masry_bedair_2013, title={Tandem InGaP/GaAs-quantum well solar cells and their potential improvement through phosphorus carry-over management in multiple quantum well structures}, DOI={10.1109/pvsc.2013.6744479}, abstractNote={InGaP/GaAs/Ge multijunction solar cell (MJSC) efficiency can be increased through improved current matching among the subcells with multiple quantum wells (MQWs) being promising for this purpose. In this study we show that InGaAs/GaAsP QWs utilizing high phosphorus composition barriers can be successfully incorporated into the GaAs subcell of an InGaP/GaAs tandem solar cell. This InGaP/GaAs-MQW device has an enhanced short circuit current density when compared to that of a standard InGaP/GaAs tandem device with minimal impact on either GaAs or InGaP subcell open circuit voltage. Additionally, phosphorus carry-over in the MQW structure is investigated through the use of photoluminescence (PL). It is demonstrated that the phosphorus carry-over can be overcome through the utilization of thick GaAs transition layers at the GaAsP→InGaAs interfaces, resulting in a MQW with an extended absorption edge.}, booktitle={2013 ieee 39th photovoltaic specialists conference (pvsc)}, author={Samberg, J. P. and Bradshaw, G. K. and Carlin, C. Z. and Colter, P. C. and Edmondson, K. and Hong, W. and Fetzer, C. and Karam, N. and El-Masry, N. A. and Bedair, S. M.}, year={2013}, pages={1737–1740} }
 @article{colter_carlin_samberg_bradshaw_bedair_2011, title={Staggered InGaAs/GaAsP strained layer superlattices for use in optical devices}, volume={208}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.201026624}, abstractNote={Strained layer superlattice (SLS) structures lattice matched to GaAs, such as InGaAs/GaAsP, use thin films to meet both the strain balance and critical layer thickness (CLT) conditions. Optical absorption in the InGaAs layers will be shifted to higher energies relative to thick films due to both the quantum size effect (QSE) and compressive strain, which is an undesirable restriction. We report on a new concept for a strain balance superlattice that will allow for thicker InGaAs films and reduce the cutoff energy. The staggered strain balanced superlattice structure can improve the performance of optical detectors and solar cells.}, number={12}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Colter, P. C. and Carlin, C. Z. and Samberg, J. P. and Bradshaw, G. K. and Bedair, S. M.}, year={2011}, month={Dec}, pages={2884–2888} }